The invention relates to computer assisted medical surgery and in particular relates to systems for displaying prerecorded visual images during surgical operations.
Presently available medical imaging techniques such as CAT (Computerized Axial Tomography), MRI (Magnetic Resonance Imaging), and PET (Position Emission Tomography), are known to be helpful not only for diagnostic purposes, but also for providing assistance during surgery. Prerecorded images may be displayed during surgical operations to provide the surgeon with illustrative reference mappings of pertinent portions of a patient's body.
Tracking systems for monitoring the position of a medical instrument have also been developed for use with image display systems. Generally, as the surgeon moves the medical instrument with respect to the patient's body, associated prerecorded images are displayed responsive to the movement of the instrument. Such tracking systems typically involve either the use of a passive articulated arm attached to the medical instrument, optical detection or ultrasonic detection.
Tracking systems using a passive articulated mechanical arm attached to a medical instrument are disclosed in U.S. Pat. Nos. 5,186,174 and 5,230,623. Generally, as the surgeon moves the surgical instrument with respect to the patient's body, micro recorders at the joints of the articulated arm record the respective amounts of movement of each arm member. The outputs of the micro recorders are processed and the position of the medical instrument with respect to the base of the articulated arm is thereby monitored. One or more prerecorded images are then displayed responsive to the movement of the surgical instrument. Such articulated arm tracking systems, however, require that the instrument be attached to a cumbersome mechanical arm. Also, although free movement of the tip of the arm in three dimensional space may be theoretically possible, the surgeon might experience difficulty positioning the instrument at certain locations and in desired orientations within the body.
Tracking systems using optical detection (video cameras and/or CCDs (Charge Coupled Devices)) have been proposed for monitoring the position of a medical instrument with respect to a reference unit as mentioned in U.S. Pat. No. 5,230,623. Such systems, however, require that the reference unit and the instrument both be within the view of the camera. This not only limits the movement of the surgical staff, but also requires that at least a portion of the medical instrument remain outside the patient's body.
Tracking systems using ultrasonic detection are generally disclosed in U.S. Pat. No. 5,230,623. Such systems, however, are disclosed to be used in a fashion similar to optical detection, i.e., triangulation of transmitted signals. The transmitted signals are sent from one or more senders to associated receiver(s), and the distances travelled by the signals are determined from either timing or amplitude changes. Again, the transmission path must remain unobstructed.
A further shortcoming common to each of the above tracking systems is that the patient must not move during the operation. Although the patient is likely to be generally anesthetized, the patient's body may be inadvertently moved by the surgical staff, or the surgeon may want to move the body for better positioning. If the body is moved after the tracking system has been initialized, then the tracking will be misaligned.
There is a need therefore for a system for monitoring the position of a medical instrument with respect to a patient's body that avoids these and other shortcomings of present devices.